Coated transparencies and transparent laminates incorporating a transparent polyurethane composition

ABSTRACT

The present invention is embodied in a polyurethane composition suitable for coating, casting or laminating, which incorporates a prescribed additive selected to increase the composition&#39;s electrical conductivity without adversely affecting the composition&#39;s transparency and percent haze and without adversely affecting its adhesion to an underlying substrate or its environmental durability, especially with regard to humidity resistance. The prescribed additive is an ionizable metal salt of a perfluoroalkylsulfonimide, in a weight percent of 0.5 to 5.0, with the metal being an alkali metal and the preferred perfluoroalkylsulfonimide being trifluoromethanesulfonimide. The most preferred salt is lithium trifluoromethanesulfonimide. The preferred polyurethanes include both aliphatic polyetherurethanes and aliphatic polyesterurethanes. Use of the additive enhances the composition&#39;s electrical conductivity by at least about two orders of magnitude, thereby minimizing the risk of a static charge building to a point where a shock hazard is created or the polyurethane coating or laminate is damaged by a rapid discharge of electrical current. The composition is particularly useful in an aircraft window or transparency, where it can be used as a coating or laminated film overlaying a transparent conductive coating.

BACKGROUND OF THE INVENTION

This invention relates generally to transparent polyurethanecompositions, and to coated transparencies and laminates incorporatingsuch compositions, and more particularly to such compositions, coatedtransparencies, and laminates having antistatic or static dissipativeproperties.

Polyurethanes and other organic polymers generally are poor conductorsof electricity. Consequently, these polymers cannot be usedsatisfactorily without modification in applications where staticdissipative properties are required.

Several methods have been used in the past to modify polyurethanes so asto increase their electrical conductivity, and thereby to betterdissipate a buildup of static charge. In one such method, conductivefibers or particles are incorporated into the polyurethane matrix. Thismethod is not suitable for use with polyurethanes that are transparent,however, because the conductive filler materials render the modifiedpolyurethane opaque.

In another method for modifying polyurethanes to increase theirelectrical conductivity, conductive polymers based on polyanilines areincorporated into the polyurethane matrix. Again, however, this methodis not suitable for use with polyurethanes that are transparent, becausethe polyaniline additives form a dispersed phase that reduces thepolyurethane's transparency. In addition, polyanilines generally areineffective at increasing the modified polyurethane's conductivity whenincorporated at a low concentration.

In yet another method for modifying polyurethanes to increase theirelectrical conductivity, hydrophilic additives such as amines andquaternary ammonium salts are used to increase the polyurethane'ssurface conductivity. These additives function by migrating to thepolyurethane's surface, where they attract water and thereby create aconductive film. This method is not suitable for polyurethane coatingsand laminates, however, because the additive also migrates to thesurface of the polyurethane that interfaces with the underlyingsubstrate, to cause a loss of adhesion. In addition, such additives canlose their effectiveness over time, because they can leach from thepolyurethane under normal use conditions.

Still other methods for modifying polyurethanes to increase theirelectrical conductivity usable in the past only for polyurethane foams,call for adding ionizable metal salts coupled with an enhancer. Thepreferred salt cation is an alkali or alkaline earth metal ion, and thepreferred anion is the conjugate base of an inorganic acid or a C2-C4carboxylic acid. The preferred enhancers are phosphate esters and saltsor esters of fatty acids.

None of these known additives for increasing the electrical conductivityof polyurethanes are considered fully satisfactory for use inpolyurethanes that are transparent, and particularly in polyurethanesthat are used as coatings or in laminates for aircraft windows.

In general, non-ionic additives and polyol modifiers have been found tosignificantly enhance electrical conductivity only if used at highlevels, which can adversely affect other important properties, such astransparency and mechanical strength. Ionic additives, includingquarternary ammonium salts and ionizable metal salts, generally are moreeffective in enhancing electrical conductivity. The most effective knownadditives of this kind are ionizable metal salts ofperfluoroalkylsulfonates. However, none of these ionic additives areconsidered filly satisfactory for use in transparent polyurethanes usedas coatings or in laminates, because with aging they can cause a loss oftransparency and a loss of adhesion.

It should therefore be appreciated that there is a need for an improvedpolyurethane composition, and for coated transparencies and laminatesincorporating such a composition, that incorporates an additive forenhancing electrical conductivity without adversely affecting thecomposition's transparency and without adversely affecting thecomposition's adhesion to an underlying substrate. The present inventionfulfills this need and provides further related advantages.

SUMMARY OF THE INVENTION

The present invention is embodied in an improved polyurethanecomposition, and in coated transparencies and laminates (e.g., aircraftwindows) incorporating such a composition, the composition incorporatinga prescribed additive for enhancing electrical conductivity withoutadversely affecting the composition's transparency and or adhesion to anunderlying substrate. More particularly, the polyurethane compositionincorporates 0.5 to 5.0 weight percent of an ionizable salt of aperfluoroalkylsulfonimide. The metal preferably is an alkali metal, andthe perfluoroalkylsulfonimide preferably is trifluoromethanesulfonimide.The most preferred ionizable salt is lithiumtrifluoromethanesulfonimide, in a weight percent in the range of 1.0 to3.0. This enhances the composition's electrical conductivity by at leastabout two orders of magnitude.

The ionizable salt of a perfluoroalkylsulfonimide is suitable for use asan additive in both polyesterurethanes and polyetherurethanes. Thepreferred polyurethane composition incorporates aliphaticpolyetherurethane, with which the prescribed additive can reduceelectrical volume resistivity to values of less than about 1×10¹¹ohm-cm.

One important use of the polyurethane composition of the invention is asa transparent coating or as part of a transparent laminate. When used asa transparent coating, the composition has particular utility whencoated onto a transparent, conductive metal coating, e.g., indium tinoxide (ITO), gold, and gold/metal oxide stacks, overlaying a transparentsubstrate. The enhanced electrical conductivity minimizes the risk thatstatic charge can build to a point where a shock hazard is created orthe polyurethane coating is damaged by a rapid discharge of electricalcurrent.

Other features and advantages of the invention should become apparentfrom the following description of the preferred embodiments, taken inconjunction with the accompanying drawings, which illustrate, by way ofexample, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph depicting the relationship between volume resistivityand temperature, for the polyurethane compositions of Examples 15 and21.

FIG. 2 is a graph depicting the relationship between volume resistivityand temperature, for the polyetherurethane compositions of Examples 20and 21.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is embodied in a polyurethane composition thatincorporates a prescribed additive selected to increase thecomposition's electrical conductivity (i.e., decrease its volumeresistivity) without adversely affecting the composition's transparencyand environmental durability, or the transparency's adhesion to anunderlying substrate. The prescribed additive is an ionizable metal saltof a perfluoroalkylsulfonimide, with the metal being an alkali metal andthe preferred perfluoroalkylsulfonimide beingtrifluoromethanesulfonimide. The most preferred salt is lithiumtrifluoromethanesulfonimide. The preferred polyurethanes include bothaliphatic polyetherurethanes and aliphatic polyesterurethanes.

The modified polyurethane composition is ideal for use as a coating fora transparent substrate or as part of a transparent laminate. Thecomposition's enhanced conductivity minimizes the possibility of thebuildup of static charge where a shock hazard is created or damage tothe polyurethane can occur.

The modified polyurethane composition of the invention can best beunderstood by reference to the following examples.

EXAMPLES 1-15

Preparation of Transparent Polyesterurethanes Incorporating AntistaticAdditives

A transparent aliphatic polyesterurethane mix, suitable for coating,laminating or casting, was prepared from the formulation set forth inTable 1.

                  TABLE 1                                                         ______________________________________                                        Aliphatic Polyesterurethane Formulation                                       Raw Material Description   Parts (wt.)                                        ______________________________________                                        Desmodur W.sup.1                                                                           Bis(4-isocyanato-                                                                           38.62                                                           cyclohexyl)methane                                               Tone 305.sup.2                                                                             Polycaprolactone triol                                                                      43.88                                              Tone 210.sup.2                                                                             Polycaprolactone diol                                                                       17.20                                              Butanediol   Chain extender                                                                              0.36                                               Tinuvin 328.sup.3                                                                          UV stabilizer 0.50                                               Irganox 1010.sup.3                                                                         Antioxidant   0.50                                               ______________________________________                                         .sup.1 Available from Bayer                                                   .sup.2 Available from Union Carbide                                           .sup.3 Available from Ciba Geigy                                         

After a homogeneous solution of the specified polyesterurethane mix wasprepared, with the solution temperature at 100° F., 30 ppm dibutyl tindilaurate catalyst and an antistatic additive were added. Fifteen suchsolutions were prepared, using the antistatic additives identified inTable 2. The resulting solutions were then cast onto glass or anothersuitable surface with a transparent conductive coating and cured at 120°F. for 12 hours, followed by 180° F. for 24 hours. The cast films wereabout 0.025 inches thick. After curing, the volume resistivity of eachof the 15 formulations was measured (at 60° F.) using the methoddescribed in ASTM D-257. The additives, their weight percentage, and theresulting electrical resistivity measurements are presented in Table 2.

                  TABLE 2                                                         ______________________________________                                        Antistatic Additives in Polyesterurethane                                                                           Vol. Res.                               Ex.  Additive      Type         Wt. % (ohm-cm)                                ______________________________________                                        1    none          --           0     >10.sup.14                              2    Cyagard SN.sup.1                                                                            Ammonium salt                                                                              1.0   1.3 × 10.sup.13                   3    Markstat AL 12.sup.2                                                                        Ammonium salt                                                                              1.0   5.0 × 10.sup.13                   4    Markstat AL 12.sup.2                                                                        Ammonium salt                                                                              4.0   1.3 × 10.sup.12                   5    Antistaticum RC100.sup.3                                                                    Polyether polyol                                                                           1.0   >10.sup.14                              6    Antistaticum RC100.sup.3                                                                    Polyether polyol                                                                           10.0  4.8 × 10.sup.12                   7    Stature II.sup.4                                                                            Polyether polyol                                                                           1.0   >10.sup.14                              8    Stature II.sup.4                                                                            Polyether polyol                                                                           10.0  9.2 × 10.sup.12                   9    Larostat 377.sup.5                                                                          Ammonium salt                                                                              1.0   1.5 × 10.sup.13                   10   KenStat KSMZ100.sup.6                                                                       Zirconate salt                                                                             1.0   1.9 × 10.sup.13                   11   Atmer 154.sup.7                                                                             Fatty acid ester                                                                           1.0   >10.sup.14                              12   Atmer 154.sup.7                                                                             Fatty acid ester                                                                           10.0  1.5 × 10.sup.13                   13   Versicon.sup.8                                                                              Polyaniline  1.0   >10.sup.14                              14   Fluorad FC122.sup.9                                                                         Lithium trifluoro-                                                                         1.0   4.5 × 10.sup.12                                      methane sulfonate                                          15   Fluorad HQ115.sup.9                                                                         Lithium trifluoro-                                                                         1.0   3.1 × 10.sup.12                                      methanesulfonimide                                         ______________________________________                                         .sup.1 Available from Cytec                                                   .sup.2 Available from Witco                                                   .sup.3 Available from Bayer                                                   .sup.4 Available from Dow                                                     .sup.5 Available from PPG                                                     .sup.6 Available from Kenrich                                                 .sup.7 Available from ICI                                                     .sup.8 Available from Allied Signal                                           .sup.9 Available from 3M                                                 

Examples 1-15 show that lithium trifluoromethanesulfonimide is the mosteffective of the identified antistatic additives in reducing volumeresistivity of a transparent polyesterurethane of the kind suitable foruse as a coating or as part of a laminate. Volume resistivity is reducedby substantially more than it is reduced by other additives, and bynearly two orders of magnitude over what it would have been without anyadditive.

The volume resistivities of the polyesterurethane compositions ofExamples 1-15 all vary inversely with temperature, thus increasing withdecreasing temperature. FIG. 1 is a graph showing the specificdependence of volume resistivity on temperature for thepolyesterurethane composition of Example 15.

EXAMPLES 16-21

Preparation of Transparent Polyetherurethanes Incorporating AntistaticAdditives

A transparent aliphatic polyetherurethane mix, suitable for coating orcasting, was prepared from the formulation set forth in Table 3.

                  TABLE 3                                                         ______________________________________                                        Aliphatic Polyetherurethane Formulation                                       Raw Material  Description       Parts (wt.)                                   ______________________________________                                        Desmodur W.sup.1                                                                            Bis(4-isocyanato- 38.06                                                       cyclohexyl)methane                                              Terathane 1000.sup.2                                                                        Polytetramethylene oxide diol                                                                   54.48                                         Trimethylol propane                                                                         Triol             7.46                                          Tinuvin 328.sup.3                                                                           UV stabilizer     0.50                                          Cyagard 1164.sup.4                                                                          UV stabilizer     0.50                                          Irganox 1010.sup.3                                                                          Heat stabilizer   0.50                                          Sanduvor 3055.sup.5                                                                         Light stabilizer  0.50                                          ______________________________________                                         .sup.1 Available from Bayer                                                   .sup.2 Available from DuPont                                                  .sup.3 Available from Ciba Geigy                                              .sup.4 Available from Cytec                                                   .sup.5 Available from Clariant                                           

After a homogeneous solution of the specified polyetherurethane mix wasprepared, with the solution temperature at 100° F., 30 ppm dibutyl tindilaurate catalyst and an antistatic additive were added. Six suchsolutions were prepared, using the antistatic additives identified inTable 4. The resulting solutions were then cast onto glass or anothersuitable surface with a transparent conductive coating and cured at 120°F. for 12 hours, followed by 180° F. for 24 hours. The cast films wereabout 0.025 inches thick. After curing, the volume resistivity of eachof the six formulations was measured (at 60° F.) using the methoddescribed in ASTM D-257. The additives, their weight percentage, and theresulting resistivity measurements are presented in Table 4.

                  TABLE 4                                                         ______________________________________                                        Antistatic Additives in Polyetherurethane                                     Ex. Additive  Type         Wt. % Vol. Res. (ohm-cm)                           ______________________________________                                        16  none      --           0     1.3 × 10.sup.13                        17  Cyagard   Ammonium salt                                                                              1.0   1.0 × 10.sup.12                            SN.sup.1                                                                  18  KenStat   Zirconate salt                                                                             Not   soluble in                                       KSMZ100.sup.2                polyurethane                                 19  Versicon.sup.3                                                                          Polyaniline  1.0   1.3 × 10.sup.13                        20  Fluorad   Lithium trifluoro-                                                                         1.0   1.9 × 10.sup.11                            FC122.sup.4                                                                             methane sulfonate                                               21  Fluorad   Lithium trifluoro-                                                                         1.0   1.7 × 10.sup.10                            HQ115.sup.4                                                                             methanesulfonimide                                              ______________________________________                                         .sup.1 Available from Cytec                                                   .sup.2 Available from Kenrich                                                 .sup.3 Available from Allied Signal                                           .sup.4 Available from 3M                                                 

Examples 16-21 show that lithium trifluoromethanesulfonimide not only isthe most effective of the identified antistatic additives in reducingvolume resistivity of a transparent polyesterurethane, but also of atransparent polyetherurethane. Volume resistivity is reduced bysubstantially more than it is reduced by other additives, and by wellmore than two orders of magnitude over what it would have been withoutany additive.

FIG. 2 is a graph showing the dependence of volume resistivity ontemperature for the polyetherurethane compositions of Examples 20 and21. Also included in FIG. 2 are curves showing the dependence of volumeresistivity on temperature for polyetherurethane compositions containing3% lithium trifluoromethanesulfonate (Fluorad FC 122), and 2% and 3%lithium trifluoromethanesulfonimide (Fluorad HQ115). It will be notedthat increasing the concentration of lithium trifluoromethanesulfonimideto 2% causes a marked reduction in volume resistivity, but thatincreasing the concentration further to 3% has little, if any,additional effect. The resistivity/temperature curve for the compositionof Example 21 is repeated in FIG. 1, so that the electrical resistivityof polyesterurethane and polyetherurethane, both containing lithiumtrifluoromethanesulfonimide (Fluorad HQ115), can be readily compared.

EXAMPLES 22-27

Durability Testing of Modified Polyurethanes

It is noted from Tables 2 and 4 that the particular additives thatyielded the largest increase in conductivity in the two types ofpolyurethanes were Fluorad FC122 (lithium trifluoromethanesulfonate) andFluorad HQ115 (lithium trifluoromethanesulfonimide). In Examples 22-27,mixes of both polyesterurethane and polyetherurethane containing theseadditives, in weight percentages of both 1% and 3%, were prepared asspecified above and then coated onto polycarbonate sheet samples thathad been pre-coated with a transparent, conductive coating and primer.

The pre-coated polycarbonate had been prepared using aircraft-gradepolycarbonate sheet (MIL-P-83310) with a conventional polysiloxane hardcoating. These sheets were first coated with a three-layer indium tinoxide (ITO)/gold/ITO film stack, using a sputtering process, to a sheetresistance of about 15 ohms/square. A silica (SiO₂) layer about 30nanometers in thickness was then deposited over the ITO coating using anevaporation process. Finally, an aminosilane primer was then applied byflow coating using a 0.1% solution of A-1100 primer (Union Carbide)dissolved in isopropanol.

After curing, the coated samples were exposed to 95%-100% relativehumidity, at a temperature of 120° F., and light transmission, haze, andadhesion were measured at 500-hour intervals. The samples incorporating3% additive concentration levels were included to represent worst-caseconditions, and to accelerate the rate of processes causing propertydegradation. The results of these tests are presented in Table 5.

                  TABLE 5                                                         ______________________________________                                        Humidity Testing of Polyurethane Coatings                                     Additive, p-urethane.sup.1                                                                Exp Time, hrs.sup.2                                                                      % LT.sup.3                                                                            % Haze.sup.3                                                                         Adhesion.sup.4                          ______________________________________                                        1% FC122, p-ester                                                                         0          82.5    1.5    100%                                                500        75.3    >50    Delami-                                                                       nated                                   1% HQ115, p-ester                                                                         0          83.1    1.4    100%                                                500        83.0    1.6    100%                                                2000       83.0    1.6    100%                                    1% FC122, p-ether                                                                         0          83.1    1.3    100%                                                500        77.2    >50    Delami-                                                                       nated                                   1% HQ115, p-ether                                                                         0          79.7    1.2    100%                                                500        79.3    1.6    100%                                                2000       78.7    1.8    100%                                    3% FC122, p-ether                                                                         0          83.3    1.0    100%                                                500        Opaque  Opaque Delami-                                                                       nated                                   3% HQ115, p-ether                                                                         0          81.8    1.5    100%                                                500        81.9    1.8    100%                                                2000       81.9    1.6    100%                                    ______________________________________                                    

Percent light transmission and percent haze were measured according toASTM D-1003, and percent adhesion was measured according to ASTM D-3359.

Examples 22-27 show the surprisingly better humidity resistance oftransparent polyurethane coatings modified to incorporate lithiumtrifluoromethanesulfonimide, as compared with the same polyurethanesmodified with lithium trifluoromethanesulfonate. In particular, thecoatings incorporating lithium trifluoromethanesulfonimide exhibitedsubstantially no degradation in light transmission, percent haze andpercent adhesion during the humidity tests. In contrast, the coatingsincorporating lithium trifluoromethanesulfonate exhibited substantialdegradation in light transmission and percent haze after just 500 hoursof the humidity tests, and the coatings all delaminated from theirunderlying substrates.

EXAMPLES 28-30

Transparent Antistatic Polyetherurethane Bilayer Coatings on ITO-CoatedGlass, Polycarbonate, and Acrylic

In Example 28, a soda lime glass surface was coated with ITO, silica,and an aminosilane primer, as described above in Examples 22-27. To forma first polyurethane coating for this glass surface, an adhesivesolution was prepared from a moisture-curable, aromaticpolyetherurethane, thermoset adhesive, based on methylene diphenyldiisocyanate (MDI) and polytetramethylene oxide polyols, incyclohexanone at 18% concentration. Lithium trifluoromethanesulfonimide(Fluorad HQ115) was then dissolved in the adhesive solution at aconcentration of 1% based on the polyurethane, and this adhesivesolution was then flow coated onto the ITO/SiO₂ -coated glass surfaceand cured at room temperature for 3-4 hours.

To form a second polyurethane coating over the first polyurethanecoating, a polyesterurethane solution was prepared as set forth in Table1, and 1% lithium trifluoromethanesulfonimide (Fluorad HQ115) and 200ppm dibutyl tin dilaurate catalyst were then added. The resultingsolution was then coated over the first polyurethane coating and curedat 100-140° F. for two hours followed by 180° F. for 24 hours. Aftercuring, the resulting bilayer coating system was determined by themethod of ASTM D-257 to have a volume resistivity of 3.2×10¹² ohm-cm.

In Example 29, ITO and SiO₂ coatings were applied to the surface of apolysiloxane-coated polycarbonate sheet using the procedure describedabove in Examples 22-27. A bilayer polyurethane coating using 1% FluoradHQ115 was then applied to the surface and cured as described in Example28. The volume resistivity of this bilayer coating system was determinedby the method of ASTM D-257 to be 3.1×10¹² ohm-cm.

In Example 30, a three-layer ITO/gold/ITO stack and a SiO₂ coating wereapplied to a polysiloxane-coated acrylic sheet, using the proceduredescribed in Examples 22-27. The acrylic sheet was aircraft grade perMIL-P-25690. A bilayer polyurethane coating using 1% Fluorad HQ115 wasthen applied to the surface and cured as described in Example 28. Thevolume resistivity of this bilayer coating system was determined by themethod of ASTM D-257 to be 3.5×10¹² ohm-cm.

Examples 28-30 show the effectiveness of a bilayer polyurethane coatingsystem using a thermoset adhesive and applied over various transparentsubstrates.

EXAMPLES 31-32

Transparent Antistatic Polyurethane Bilayer Laminate

In Example 31, a first polyurethane sheet was prepared by forming asolution at 100° F. based on the formulation set forth in Table 6. Addedto this solution was 1% Fluorad HQ115, followed by 200 ppm dibutyl tindilaurate catalyst. The mix was then pumped into a cell formed from twoglass sheets and a peripheral gasket, and cured at 180° F. for 24 hours.After separation from the glass, a transparent polyurethane sheet ofabout 0.040 inches thickness was obtained. The volume resistivity ofthis cast thermoset sheet was determined by the method of ASTM D-257 tobe 4.9×10¹² ohm-cm.

                  TABLE 6                                                         ______________________________________                                        Aliphatic Polyesterurethane Formulation                                       Raw Material Description   Parts (wt.)                                        ______________________________________                                        Desmodur W.sup.1                                                                           Bis(4-isocyanato-                                                                           47.0                                                            cyclohexyl)methane                                               Tone 301.sup.2                                                                             Polycaprolactone triol                                                                      22.0                                               Tone 200.sup.2                                                                             Polycaprolactone diol                                                                       30.0                                               Tinuvin 328.sup.3                                                                          UV stabilizer 0.5                                                Irganox 1010.sup.3                                                                         Antioxidant   0.5                                                ______________________________________                                         .sup.1 Available from Bayer                                                   .sup.2 Available from Union Carbide                                           .sup.3 Available from Ciba Geigy                                         

A second polyurethane sheet was then prepared by dissolving FluoradHQ115 in isopropanol, at a concentration of 1%, and this solution wasthen sprayed evenly onto both surfaces of a 0.025-inch thick extrudedtransparent aliphatic polyetherurethane sheet (PE-399 manufactured byMorton). Sufficient solution was applied to yield a total concentrationof 1% Fluorad HQ115 based on the polyurethane sheet. The sheet was airdried for 16 hours, to evaporate all of the solvent. The volumeresistivity of this modified sheet was determined by the method of ASTMD-257 to be 1.5×10¹⁰ ohm-cm.

The first and second polyurethane sheets described above were thenlaminated to a polycarbonate sheet having a conductive ITO/gold/ITOstack and SiO₂ coatings, as described above in Examples 22-27.Specifically, the second polyurethane sheet, i.e., the thermoplasticaliphatic polyetherurethane containing 1% Fluorad HQ115, was used as anadhesive between the polycarbonate sheet and the first polyurethanesheet, i.e., the polyesterurethane sheet. The three sheets werelaminated in an autoclave using conventional vacuum bag techniques, at atemperature of 200° F. and pressure of 100 psi. The volume resistivityof this bilayer laminate was determined by the method of ASTM D-257 tobe 1.2×10¹² ohm-cm.

In Example 32, a laminate was prepared as described above in Example 31,except that the first polyurethane sheet was formed of polyetherurethane(PE-399, Morton), with Fluorad HQ115 incorporated into the resin priorto extrusion at a 1% level. The volume resistivity of the resultingextruded sheet was determined by the method of ASTM D-257 to be 1.8×10⁹ohm-cm. The second polyurethane sheet and the coated polycarbonate sheetwere the same as in Example 32, and the various layers were laminatedtogether in the same manner as was done in Example 32. The volumeresistivity of this bilayer laminate was determined by the method ofASTM D-257 to be 1.5×10¹² ohm-cm.

Examples 31 and 32 show the effectiveness of the polyurethanes of theinvention used as bilayer laminates with a thermoplastic adhesive. Theexamples also show alternate methods of incorporating the preferredantistatic additive into transparent polyurethanes.

It should be appreciated from the foregoing disclosure that the presentinvention provides an improved polyurethane composition thatincorporates a prescribed additive for enhancing electrical conductivitywithout adversely affecting the composition's transparency and oradhesion to an underlying substrate. The additive enhances thecomposition's electrical conductivity by at least about two orders ofmagnitude, making it ideally suited for use as a coating for atransparency, or as part of a laminate, that can avoid shock hazards ordamage from a buildup of static charge.

Although the invention has been disclosed with reference only to thepresently preferred embodiments, those of ordinary skill in the art willappreciate that various modifications can be made without departing fromthe invention. Accordingly, the invention is defined only by thefollowing claims.

We claim:
 1. A coated transparency comprising:a transparent substratehaving an outer surface; a thin, transparent conductive coatingoverlaying the substrate's outer surface; and a transparent polyurethanecoating overlaying the transparent conductive coating, the polyurethanecoating formed from a composition incorporating 0.5 to 5.0 weightpercent of an ionizable salt of a perfluoroalkylsulfonimide.
 2. A coatedtransparency as defined in claim 1, wherein the transparent polyurethanecoating is an aliphatic polyesterurethane.
 3. A coated transparency asdefined in claim 1, wherein the transparent polyurethane coating is analiphatic polyetherurethane.
 4. A coated transparency as defined inclaim 1, wherein the metal in the transparent polyurethane coating is analkali metal and the perfluoroalkylsulfonimide in the transparentpolyurethane coating is trifluoromethanesulfonimide.
 5. A coatedtransparency as defined in claim 4, wherein the ionizable salt in thetransparent polyurethane coating is lithium trifluoromethanesulfonimide.6. A coated transparency as defined in claim 5, wherein the weightpercent of lithium trifluoromethanesulfonimide in the transparentpolyurethane coating is in the range of 1.0 to 3.0.
 7. A coatedtransparency as defined in claim 1, wherein the ionizable salt in thetransparent polyurethane coating is incorporated at a concentrationeffective to reduce the coating's volume resistivity, after curing, bymore than about two orders of magnitude over what it would be in theabsence of the ionizable salt.
 8. A coated transparency as defined inclaim 1, wherein the transparent polyurethane coating comprises:atransparent underlayer incorporating a thermoset adhesive; and atransparent overlayer.
 9. A coated transparency as defined in claim 8,wherein:the underlayer of the transparent polyurethane coating is formedfrom a solution of a moisture-curable, aromatic polyetherurethane,thermoset adhesive, based on methylene diphenyl diisocyanate andpolytetramethylene oxide polyols, in cyclohexanone, incorporating theionizable salt of a perfluoroalkylsulfonimide; and the overlayer of thetransparent polyurethane coating is an aliphatic polyesterurethaneincorporating the ionizable salt of a perfluoroalkylsulfonimide.
 10. Acoated transparency as defined in claim 1, wherein:the transparentsubstrate is selected from the group consisting of polycarbonate, glass,and acrylic; and the tin, transparent conductive coating is selectedfrom the group consisting of gold, indium tin oxide, metal oxide, andmulti-layer stacks thereof.
 11. A coated transparency comprising:atransparent substrate having an outer surface; a thin, transparentconductive coating overlaying the substrate's outer surface; and atransparent aliphatic polyetherurethane coating overlaying thetransparent conductive coating, wherein the polyetherurethane coating isformed from a composition incorporating 1.0 to 3.0 weight percent oflithium trifluoromethanesulfonimide, and wherein the polyetherurethanecoating, after curing, has a volume resistivity of less than about1×10¹¹ ohm-cm.
 12. A transparent laminate comprising:a transparentsubstrate having an outer surface; a thin, transparent conductivecoating overlaying the substrate's outer surface; and a transparentpolyurethane layer laminated to the transparent conductive coating, thepolyurethane layer formed from a composition incorporating 0.5 to 5.0weight percent of an ionizable salt of a perfluoroalkylsulfonimide. 13.A transparent laminate as defined in claim 12, wherein the transparentpolyurethane layer incorporates an aliphatic polyesterurethane.
 14. Atransparent laminate as defined in claim 12, wherein the transparentpolyurethane layer incorporates an aliphatic polyetherurethane.
 15. Atransparent laminate as defined in claim 12, wherein the metal in thetransparent polyurethane layer is an alkali metal and theperfluoroalkylsulfonimide in the transparent polyurethane layer istrifluoromethanesulfonimide.
 16. A transparent laminate as defined inclaim 15, wherein the ionizable salt in the transparent polyurethanelayer is lithium trifluoromethanesulfonimide.
 17. A transparent laminateas defined in claim 16, wherein the weight percent of lithiumtrifluoromethanesulfonimide in the transparent polyurethane layer is inthe range of 1.0 to 3.0.
 18. A transparent laminate as defined in claim12, wherein the ionizable salt in the transparent polyurethane layer hasa weight percentage effective to reduce the layer's volume resistivity,after curing, by more than about two orders of magnitude over what itwould be in the absence of the ionizable salt.
 19. A transparentlaminate as defined in claim 12, wherein:the transparent polyurethanelayer comprisesa transparent underlayer functioning as an adhesivelayer, and a transparent overlayer; and both the underlayer and theoverlayer of the transparent polyurethane layer incorporate theionizable salt of a perfluoroalkylsulfonimide.
 20. A transparentlaminate as defined in claim 19, wherein:the underlayer of thetransparent polyurethane layer is a polyetherurethane sheet havingsurfaces carrying the ionizable salt of a perfluoroalkylsulfonimide; andthe overlayer of the transparent polyurethane layer is an aliphaticpolyesterurethane incorporating the ionizable salt of aperfluoroalkylsulfonimide.
 21. A transparent laminate as defined inclaim 12, wherein:the transparent substrate is selected from the groupconsisting of polycarbonate, glass, and acrylic; and the thin,transparent conductive coating is selected from the group consisting ofgold, indium tin oxide, metal oxide, and multi-layer stacks thereof.